incorrect
applies to both single and double breasted jackets
jackets with one button = always fastened
two buttons = always, never (top to bottom)
three buttons = sometimes, always, never
four buttons or more (just plain silly but still) = just button the top three and leave the bottom undone

double breasted is different depending on the lapels of the jacket, but the rule still applies if there are three sets that it is sometimes, always, never (top to bottom)

The jacket is specifically cut so that a single button (the top) is to be fastened. The only time you would ever button more than one is if the suit jacket has three buttons and even then only the top two.

that is not how suits are designed though
it just looks plain wrong if both buttons are fastened because it pulls it too tight
like that anon said, suit jackets are specifically cut to look normal when only one button is fastened
though he is incorrect with three buttons. top to bottom, it is sometimes, always, never

I know fashion, but there's a fine line between "fashion" and "functional" that's like jackets and pants that have zippers that you would expect to open up to a pocket, but really are just decoration. just why... that's a waste of material.